The "Childmyths" blog is a spin-off of Jean Mercer's book "Thinking Critically About Child Development: Examining Myths & Misunderstandings"(Sage, 2015; third edition). The blog focuses on parsing mistaken beliefs that can influence people's decisions about childrearing-- for example, beliefs about day care, about punishment, about child psychotherapies, and about adoption.
See also http://thestudyofnonsense.blogspot.com

For worried parents, one of the real problems is
that they don’t actually know what “autistic movements” look like. Another is
that infants are individuals who often don’t conform to averages of development
that parents read in “the books” of norms for developmental milestones; they
reach some milestones earlier than the average, then turn around and act a bit
delayed on others. In addition, neither autistic or typically-developing
children are “all autistic” or “all typical”, and their behaviors overlap quite
a bit, especially during the toddler and early preschool periods. A fourth, and
very real, problem is that it’s still not understood how autism can be
predicted accurately for children who are still under a year old-- but this is exactly the age period when
parents are most likely to start agonizing and watching for those “red flags”.

Here is a very fine video that shows behavioral
differences between typically-developing toddlers and same-age children who are
later diagnosed as autistic:

When you watch this, please notice that all the
children are over 12 months of age. Don’t try to generalize from this
information to younger infants. One of the features of autism is developmental
delay, so an autistic child may act in some ways much like a younger
child-- it’s important to realize that
this cannot be reversed to mean that the younger child must also be autistic,
or to assume that developmentally appropriate behaviors of the infant are signs
of autism.

The first concept covered in the Kennedy Krieger
video is the child’s ability to use play in ways that include other people, not
necessarily as a formal “game”, but as part of a social interaction like
pretending to eat or to feed another person. The typically-developing child in
the video offers a “bite” to an observer and takes a “bite” himself. This
ability develops after about a year of age, and its absence in a younger infant
would be no reason for concern. He also follows what an adult does, imitating
in a meaningful rather than a mechanical way. The same-age child who is showing
signs of autism does none of those things, and he does not respond to his name,
which is unusual at this age (though it would happen much more frequently with
children some months younger). Although he acts as if he likes to be tickled by
his mother, he doesn’t respond socially.

The second point is the making of social connections
by typically-developing children, by looking at people and responding to
gestures. The little boy in the video even makes an effort to turn and look at
his mother, and he looks at an item pointed at by an adult, continuing the “conversation”
by pointing at it again later. Another boy of the same age shows symptoms of
autism by flapping his hands in excitement, but without looking at the adult; a typically-developing child may also
flap his hands, but he looks at adults while he does so and seems to use the
gesture as communication. The autistic child does not look at an object an
adult points at, but instead looks at the adult’s finger (as a much younger
infant might do).

The third important point in the Kennedy Krieger
video is the typically-developing child’s capacity to carry out and enjoy
social communication. The child in the video is fascinated by a moving toy, but
frequently looks at adults and then back at the toy, while smiling. When the
toy stops moving, she wordlessly communicates to the adult that she needs help
to start it again. An autistic child of the same age watches the toy carefully,
but does not smile at adults or look at them to create social communication. He
does not respond to a gesture by which an adult asks to have the toy. His mouth
and torso become tense and stiff as he handles the toy.

Watching this video may give you some hints about
differences in behavior between typically-developing and less typical children,
but remember, these differences do not
apply to children under a year of age. There may be differences at earlier
ages, but they are not yet well understood. In addition, remember that typically-developing
preschool children sometimes do “autistic” things, and autistic children
sometimes do “typical” things. It’s the general pattern of behavior that is of
importance. Finally, keep in mind that even the children who are diagnosed as on
the autistic spectrum as toddlers and preschoolers may well look much more typical
as time goes on.

About Me https://en.wikipedia.org/wiki/Jean_Mercer

Jean Mercer has a Ph.D in Psychology from Brandeis University, earned when that institution was 20 years old (you do the math). She is Professor Emerita of Psychology at Richard Stockton College, where for many years she taught developmental psychology, research methods, perception, and history of psychology. Since about 2000 her focus has been on potentially dangerous child psychotherapies, and she has published several related books and a number of articles in professional journals.
Her CV can be seen at http://childmyths.blogspot.com/2009/12/curriculum-vitae-jean.mercer-richard.html.